The fabrication of computer chips from silicon wafers requires hundreds of processing steps including sequential deposition, etching, temperature conditioning and cleaning. The ability to provide uniform conditions over the workpiece surface is crucial in the fabrication process. This is particularly the case where larger wafers having smaller feature sizes are being processed. The low gas pressures favored in processing wafers tends to promote a higher degree of non-uniformity in conventional stagnation flow reactors. While reactors operating in a stagnation flow regime can, in theory, provide the required radially uniform conditions over the surface of a semiconductor workpiece, current stagnation flow reactor design practice requires that uniformity of reaction be approached through trial and error, usually starting with a previous reactor chamber design.
It is well known to those skilled in the art that certain flow configurations have important similarity properties that render their analysis radially independent. Included in this set is stagnation flow. Given that a uniform velocity, uniform temperature and uniform composition inlet flow issues from a manifold a fixed distance above a parallel fixed solid surface which is at uniform temperature, it can be shown that the heat and mass flux to the solid surface will be everywhere uniform regardless of the radial extent of the system. Thus, the gas phase species and temperature profiles are independent of radius. The inherent radial uniformity of a stagnation flow geometry provides an important means for achieving uniform species and heat fluxes to large surface areas. Thus, the use of a stagnation flow geometry offers a means to uniformly clean, etch and deposit to surfaces.
Because the only relevant spatial coordinate for species concentration and temperature is the axial distance between the origin of the reactant gas flow and the position of the solid surface, stagnation flow offers numerous advantages insofar as a means for improving uniformity of distribution of reactants over large surface area. A more complete discussion of stagnation flow reactors is contained in co-pending application Ser. No. 08/302,155, incorporated herein by reference.
Most single wafer reactors are based on a stagnation flow geometry, in which gases are forced to flow uniformly and perpendicularly to the wafer surface. This geometry would theoretically produce uniform wafer conditions for an infinitely large mass flow of reactant gas and a wafer radially infinite in extent. For practical wafer sizes, reactor sizes and pressures, and large gas mass flow rates, uniformity is achieved over most of the wafer surface, with non-uniformities limited to a small region around the wafer's edge. However, as the pressure within the wafer reactor is lowered, gas mass flow rates are correspondingly lowered and non-uniformities in reaction rates develop between the central region of the wafer and the edge of the wafer because of imbalances in convection and diffusion of reactive species at the wafer surface, i.e., reactive species are depleted at a different rate at the central region of the wafer surface than at the edge of the wafer. At high pressures, convection can effectively counteract these imbalances. However, at low pressures gas mass flow rates are limited and diffusion forces become important.
Conventional stagnation flow configurations, which work very well at high pressure and/or high flow rates, have limitations which restrict their usefulness at low pressures and/or low flow rates. Simple scaling from high pressure designs does not produce an optimal design and generally, many iterations are required to achieve satisfactory etch uniformity across the surface of the wafer. At pressures in the range of .apprxeq.1 Torr non-uniformities, resulting from non-uniform surface reaction, become very large for a stagnation flow configuration due to the increased importance of diffusion relative to convection.
The invention described herein sets forth a method and apparatus wherein a proper geometric configuration within a low pressure or low flow rate stagnation flow reactor provides enhanced reaction uniformity.